This invention relates to a process for the preparation of sand cores for metal castings in the foundry industry.
In a particular aspect this invention relates to a 100% solids, essentially non-polluting binder for the sand.
In foundry practice, metal casting is conventionally carried out by mixing an aggregate, e.g. sand, with a foundry sand binder capable of hardening the aggregate, tamping it into a mold to form a sand core and letting it stand for a period of time to allow the binder to set up whereby the sand core becomes self-supporting, and then removing the mold. The molten metal is subsequently poured into the sand core and when cool, the sand core is broken away. The resulting casting is then finished by grinding away the imperfections.
Many materials have been used as the binder in preparing such cores. Some binders are of a class of no-bake, or cold setting resins, i.e. designed to be cured without the application of heat, while others are of a class suitable for "hot box" application. Hot box means foundry operations wherein the resin binders are cured by the application of heat. Alkyd resins have been widely used as the binder. These resins are too viscous to be used alone however. It has therefore been necessary to dilute them with a solvent so that they can be adequately mixed with the sand. The sand core must then be baked to set the alkyd and volatilize the solvent.
This process for preparing the sand core is in widespread use but it presents several problems of considerable magnitude. Chief of these is the danger of fire from the solvent. Although the core is sometimes baked before the molten metal is poured into it, there is frequently considerable residual solvent, especially where large castings are being made. Consequently flash fires are common in foundries.
Another common problem is air pollution in the foundry atmosphere due to the emission of noxious fumes from decomposition of the binder resulting from the heat of the molten metal. Large quantities of irritating white smoke are common in foundries during the pouring step. Although exhaust fans and ventilation are provided, the workers closest to the operation are exposed and furthermore the air outside the foundry is polluted by the exhaust.
J. J. Engel et al. discussed other problems and disclosed a solution thereto in U.S. Pat. No. 3,255,500, which is incorporated herein by reference thereto. Engel et al. disclosed a binder composition consisting of drying oils containing ethylenic unsaturation such as the hydrocarbon polymers and copolymers prepared from butadiene, styrene, cyclopentadiene and the like, but preferably the alkyd resins having a hydroxyl value of at least 25. The drying oil is used in combination with isocyanates and solvents preferably with a catalyst.